1. Field of the Invention
The present invention relates generally to a multiple window having a plurality of horizontal window frames, and more particularly to a joint structure for such multiple window including a mullion disposed between vertical frame members of two adjacent window frames to join them together.
2. Description of the Related Art
Multiple windows of the type concerned are known, in which a vertical frame member of one of two adjacent window frames is joined by a mullion to a vertical frame member of the other window frame. One such known joint structure is disclosed in Japanese Utility Model Laid-open Publication No. 63-138390.
The disclosed joint structure includes a mullion composed of a body and a cover member. The body has a pair of first engagement recesses on its interior side and a pair of second engagement recesses at an intermediate portion thereof. The first and second engagement recesses are fitted with corresponding pairs of engagement fins projecting from outer side surfaces of the vertical frame members to preassemble the mullion body to the vertical frame members. Then the mullion body is secured by screws to the vertical frame members, and the cover member is attached by screws to the exterior side of the vertical frame members. The vertical frame members of the two adjacent window frames are thus joined together by means of the mullion.
With the known joint structure thus constructed, partly because the engagement recesses in the mullion body are fitted with the engagement fins of the vertical frame members, and partly because the cover member is screwed to the vertical frame members, the vertical frame members and the mullion are not relatively displaceable in a horizontal direction. With the horizontal relative movement thus prohibited, when an upper or a lower horizontal frame member of one window frame thermally expand or contracts, such thermal deformation cannot be accommodated or canceled out and rather brings about a bending or deformation of the vertical frame member of the same window frame.